How We Found the First of the Fresh Impact Sites
that Formed During the Mars Global Surveyor Mission

MGS MOC Release No. MOC2-1611, 6 December 2006

The Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) has found that
meteorites are hitting the martian surface and forming new craters all the time. If you
were living on Mars, chances are that within 10 or 20 years, an impact would occur close
enough to where you live that you’d notice it—perhaps you’d hear the impact
and it would startle you out of your seat.

A year ago, it had not occurred to us, the MGS MOC science operations team, that we could
find places on Mars where meteorites had impacted the surface during the course of the MGS
mission. Such craters, if they were forming at all, would be a few meters to a few tens of
meters across; much to small to notice (or so we thought) in our MOC wide angle camera coverage.
But, on 9 January 2006, we began to realize that not only could we find
such craters, we might also be able to characterize the
present-day impact cratering rate on Mars. Surveying for fresh craters formed during
the MGS mission would provide the first direct observation—for any body in the Solar System, including
Earth and its Moon—of the present-day cratering rate, which in turn can help
test models used all the time by members of the scientific community to estimate the
age of features on planetary surfaces.

The fact that we made the discovery described here, and then went on to conduct a survey for
fresh impact craters and completed it within a few months, is a testament to the foresight of
NASA to grant MGS several mission extensions. The Primary Mission began in March 1999 and ran
through January 2001. After that, and given the health of the spacecraft and its instruments,
NASA decided it could afford to fund the mission for 1 more Mars year. That first extended
mission led to three more 1-Mars-year extensions (each nearly 2 Earth years long), the most
recent was granted at the start of October 2006.

The first fresh impact site, shown on this page, was first noticed
on 9 January 2006 in an image acquired 3 days earlier, MOC image S14-00672. That image
(top, right) was a context frame for MOC narrow angle image S14-00671. It was acquired by
MOC’s red wide angle camera at its highest possible spatial resolution, about
240 meters (262 yards) per pixel. To the northwest of the area imaged by the narrow angle
camera, the red wide angle context frame showed a dark spot. This spot was not present in
any previous image acquired by any spacecraft, from Mariner 9 (arrived in 1971) on down
through Mars Express (arrived in 2003).

Malin Space Science Systems (MSSS)
Staff Scientist Ken Edgett noticed the dark spot immediately and thought
it might be the site of a fresh impact crater because it resembled a dark spot seen at
the same scale at a known impact site suspected of being less than 25 years old
(see “New Crater Might Have Formed During The 1980s”). Here is what Edgett’s notes on 9 January 2006
said about it:

S14-00672 red WA context; dark spot at s=130,l=295 does not appear in
any previous red WA images, nor in the THEMIS VIS images that cover the
area: V14459015 and V12924012-- the former VIS image is from March 2005---
could this dark spot be a very fresh impact crater, one that formed since
March 2005???
(K. S. Edgett, 9 January 2006)

This was a process of discovery. Mike Malin, MSSS President and Chief Scientist
and also the MOC Principal Investigator, did not at first realize that this could be the
site of a fresh impact. His notes included mention of the 6 January narrow angle image (MOC S14-00671)
and its context frame (S14-00672), and wondered why the dark spot had not been targeted in the
first place:

S14-00671; S14-00672c Odd image pair: NA is of a small (~1 km)
moderately young impact crater that is relatively nondescript, but why
not the compact dark spot NW of that crater (see the context image)?
(M. C. Malin, 9 January 2006)

Of course, the dark spot had not been targeted because no one knew that it
was there. It was not evident in any previous images of Mars.
The same day that Edgett noticed the dark spot, although his notes did not reflect
this, he also thought, “What if this is the shadow of one of the moons?”
But he realized that the shadow of Phobos usually appears to be quite elliptical
(see “MOC Views of Martian Solar Eclipses”),
while the dark spot observed in the new MOC red wide angle image was more
circular. A quick check by several co-workers at MSSS showed that Phobos was on the
opposite side of the planet when the new MOC image was acquired, and Deimos is too far
away and too small to cast such a shadow.

The possibilities that remained to be explored were that either the new dark spot was
the site of a fresh impact crater, or a place where something else had changed,
such as the removal of dust by wind to make a group of dark wind streaks. The way to
determine the cause of the dark spot was simple: rotate the MGS spacecraft such that it
points the MOC high resolution camera at the dark feature, then take a picture. Employed routinely during the
MGS extended mission since early 2001, and called a Roll-Only Targeted
Observation (ROTO), the spacecraft pointed the camera at the feature on 20 February 2006.
The result was a 1.5 meters (4.9 feet) per pixel view of a small cluster of dark craters
near the center of a larger, darkened zone. One prominent dark ray pointed from the center
of the impact site toward the southwest. The new dark spot was found, indeed, to be the
site of a fresh impact.

To get a higher resolution view of the impact site, a compensated Pitch and Roll
Observation (cPROTO) image was acquired a few weeks later on 16 March 2006. The cPROTO
technique was developed in 2003. The new cPROTO image of the site has
a spatial resolution of about 0.5 meters (1.6 feet) per pixel in the downtrack
dimension, 1.5 meters (4.9 feet) in the crosstrack dimension, and thus provides an improved
view of a portion of the impact feature.

Meanwhile, as these ROTO and cPROTO data were being acquired, the MOC science operations
team also began a campaign to re-image the dust-mantled areas of Amazonis, Tharsis, and Arabia Terra
using the red wide angle camera. As explained in an accompanying release
(see “Procedure for Finding New Impact Sites on Mars Using the Mars Global Surveyor Mars Orbiter Camera”), this was the start of an effort to identify
new craters that formed on Mars since May 1999, and then to use these to characterize the
planet’s present-day cratering rate.

By examining other images of the impact site—acquired by Mars Odyssey’s
Thermal Emission Imaging System (THEMIS) as well as MGS MOC
and the Mars Express
High Resolution Stereo Camera (HRSC)—we
found that we could constrain the time when the impact occurred to a period between
12 November 2004 and 6 January 2006. On 12 November 2004, THEMIS acquired infrared
image I12924012, and it showed that the impact had not yet occurred. A later image,
THEMIS I14771016, was obtained on 13 April 2005, but it was somewhat noisy and thus
we could not be certain whether the impact occurred before or after 13 April 2005.
What is certain is that the March 2005 THEMIS VIS image mentioned in Edgett’s
9 January 2006 notes did not overlap the site, so the notes written hastily that
day were in error on that point. The MOC image obtained on 6 January 2006, thus,
was the first to show that the impact had occurred.

(A) The first figure (above, top) shows two MOC red wide angle camera context
images. The first was taken on 9 June 2001, several years before the impact occurred. The
second is the “discovery” image, acquired on 6 January 2006. In both cases, a white
box indicates the location of the MOC narrow angle image for which the context image
was obtained. For scale, the white boxes are 3 kilometers (1.9 miles) wide.

(B) The second figure (above, bottom) is a composite of sub-frames of MOC images S15-02128 and
S16-01426. The S15 image was acquired as a ROTO, the S16 as a cPROTO. The cPROTO image
has the higher spatial resolution, but did not cover the entire impact site. North is up in this
map-projected view. The single, broad dark streak that emanates from the impact site and points
toward the southwest (lower left) may indicate either the direction that the meteor came from,
or its opposite. If it represents the direction that the impactor came from, then
the streak results from disruption of dust on the martian surface as the object
came in; if the opposite, then it represents the direction that material was blasted
from the impact site, away from the direction that the meteor came. In either case, the
impactor came in at a somewhat oblique angle, and broke up just before hitting the
ground, because it formed multiple small craters. The 300 meter scale bar also represents
328 yards (984 feet).

Malin Space Science Systems and the California Institute of Technology
built the MOC using spare hardware from the Mars Observer mission.
MSSS operates the camera from its facilities in San Diego, California.
The Jet Propulsion Laboratory's Mars Surveyor Operations Project
operates the Mars Global Surveyor spacecraft with its industrial
partner, Lockheed Martin Astronautics, from facilities in Pasadena,
California and Denver, Colorado.